For many years, transport aircraft have been equipped with on board lavatories and associated holding tanks for holding effluent from the lavatories. It is important that the level of effluent in the holding tanks be monitored.
Measuring the level of effluent in containers such as aircraft lavatory waste or holding tanks has traditionally been plagued by problems with the reliabilty of the empty and full sensors. A typical conventional design used an access-port-mounted flat plate sensor device. This sensor device was essentially one plate of a capacitor that sensed the capacitance change as the liquid level rose across the sensor's surface. However, solid waste contaminants would build up on the sensor's surface, thus masking the sensor's responses. Some conventional designs even included washer systems to rinse the sensor's surface when the tank was emptied, but contamination still occurred.
It is also known that the level of fuel in a tank can be detected by vibrating the wall of the structure and measuring the response. However, such vibrating sensors appear to have little advantage over a capacitance sensor as far as contamination is concerned.
Accordingly, a need exists for a liquid level detector that will operate accurately in a contaminated environment.
To achieve this capability, the detector should be able to determine when it is so contaminated that its accuracy is impaired and then clean itself.